1. Field of the Invention
This invention relates to the field of semiconductor device interconnection contacts, and particularly to methods of fabricating such contacts so as to reduce mechanical stress.
2. Description of the Related Art
Precision analog integrated circuits (ICs) have always been sensitive to mechanical stresses encountered during the fabrication and assembly process. Such stresses can alter the electrical performance of otherwise identical devices. For example, two transistors forming a differential pair will ideally have identical electrical characteristics. However, mechanical stress—particularly in close proximity to the transistors' p-n junctions—can cause the two transistors to react differently to identical stimuli, thereby degrading the accuracy with which the circuit can operate.
One source of mechanical stress is the metallization that provides interconnections to the IC's active devices. For many device processes, the metal used to provide interconnections is aluminum based, typically aluminum copper (AlCu). Stress is built up in the Al film during device processing, and differential stress is added to the film during subsequent processing steps, especially during the cool down period following anneal. Interconnect Al in close proximity to an active device region, such as the base-emitter junction of a bipolar transistor (BJT), can cause instability in the device when subjected to temperature treatment. This can result in poor matching between active devices which are otherwise identical.